The pattern of extractions for orthodontic reasons (when they're needed) can vary a great deal and is strictly individual. One reason we might extract different, non-matching teeth on the two sides is, for example, an asymmetrical bite. On the side where space is lacking much more, a tooth further forward is extracted (for example, the fourth), and on the side with less crowding, one further back (for example, the fifth).
Another reason for asymmetrical extractions may be that on one side there is a completely healthy fourth tooth and a non-vital (root-treated) fifth tooth. In this case we will choose the fifth tooth, even if the fourth is better placed for solving the problem. If on the other side both premolars are equally healthy, we'll choose the one in the more suitable position for the treatment – for example, the fourth. The symmetry of the teeth will be preserved by carrying out the treatment plan correctly and distributing the space from the extracted teeth properly.
This answer is for general information. Your own case needs an examination by an orthodontist.